The idea to create a working tool, which manage completely by itself, such as a robot lawn mover or a robot vacuum-cleaner, is old. However, it has taken a long time before such kind of tool has reach the market The solar cell driven lawn mover, called Solar Mower, is an example of that kind of product. It cuts the grass within a border cable, which has been placed in order to fence off the cutting area. Preferably the border cable is excavated into the ground. A signal generator feeds the border cable with current, whose magnetic field affects a sensing unit on the working tool. The sensing unit detects the intensity of the magnetic field and this intensity will increase when the working tool is coming closer and closer to the border cable. The microcomputer in the lawn mover is so programmed that the lawn mover reverses when a certain signal intensity has been achieved during the increasing of the signal intensity that occurs when the sensing unit is approaching the border cable. The lawn mover will as said move backwards from the border cable and then turn and begin to cut in a direction away from the border cable. Consequently, the lawn mover turns when the powerful magnetic field at the border cable has caused a signal intensity of a certain degree. On the other hand, the microcomputer cannot in any way separate the magnetic field on the outside from that on the inside of the border cable. It means that if the lawn mover should slide outside the border cable, or be pushed outside the border cable, the lawn mover will remote from the border cable in the wrong direction, i.e. out from the cutting area. However it stops after approximately 4 metres since the picked up signals then have dropped so much that the machine stops and sends out an alarm signal. Considering the great disadvantage it means to come outside the border cable the lawn mover must therefore change direction already before the sensing unit reaches the border cable. The strength of the signals going from the lawn mover's sensing unit and further into its microcomputer depends on a number of factors, such as spreading of components, sling geometry, excavation depth, iron materials in the ground etc. etc. All this contributes to that a relatively large margin must be given as to how close the sensing unit may come to the border cable, or rather more correctly, at which signal intensity the lawn mover should turn. At cutting work the result will be an uncut edge, which can be several decimetres broad. The problem will be especially large when the cable cannot be placed outside the lawn, for instance at a house ground, an asphalt walk or a flower bed. In case the tool should be pushed outside the border cable by a child or a domestic animal, this will inevitably lead to that the tool stops and sends out an alarm signal, and then it must be lifted in or pushed back inside the border cable.